As well known, the electric power steering apparatus are steering assisting apparatus which are designed to drive an electric motor as a human operator or driver manually operates a steering wheel, during driving of a motor vehicle, to thereby assist the driver's manual steering effort. In such electric power steering apparatus, the steering assisting motor, which provides a steering torque assist (or steering assist torque), is driven by a motor control device in accordance with a PWM (Pulse Width Modulation) scheme, using a steering torque signal generated by a steering torque detector section detecting steering torque that is produced on the steering shaft by driver's operation of the steering wheel and a vehicle velocity signal generated by a vehicle velocity detection section detecting a traveling velocity of the vehicle, so as to reduce manual steering force to be applied by the human driver.
FIG. 38 is a view showing an overall setup of a typical example of the conventional electric power steering apparatus. This electric power steering apparatus 100 includes a steering torque detector section (torque sensor) 102 for detecting steering torque applied by a human driver via a steering wheel 101, an electric steering assisting motor 103 for providing an steering torque assist to the steering by the driver, a power transmission device 104 for boosting the rotational torque of the motor 103, a control device 106 for controlling operation of the motor 103 on the basis of output signals from the torque detector section 102 and vehicle velocity detector section 105, and a rack and pinion mechanism 108 for varying a direction of steerable front road wheels 107.
The electric power steering apparatus 100 is constructed to provide the steering assist torque to an upper steering shaft 109a etc. connected to the steering wheel 101. The upper steering shaft 109a is also connected at its lower end to a lower steering shaft 109b via a universal joint 109c and connected at its upper end to the steering wheel 101. The lower steering shaft 109b is operatively connected at its lower end to a pinion gear 110 meshing with a rack gear 111a provided on a rack shaft 111. The pinion gear 110 and rack gear 111a together constitute a rack and pinion mechanism 108. Tie rods 112 are connected to axial opposite ends of the rack shaft 111, and the front road wheels 107 are connected to respective outer ends of the tie rods 112. The steering assisting motor 103 is operatively connected to the lower steering shaft 109b via the power transmission mechanism 104. The power transmission mechanism 104 comprises a worm gear 104a and worm wheel 104b. The motor 103 generates steering assist torque that is delivered via the power transmission mechanism 104 to the steering shafts 109b and 109a. The steering torque detector section (torque sensor) 102, which is provided on the lower steering shaft 109b, detects steering torque applied to the steering shafts 109a and 109b through driver's operation of the steering wheel 101. The vehicle velocity detector section 105 detects a traveling velocity of the vehicle, and the control device 106 is implemented by a computer. The control device 106 receives a steering torque signal T output from the torque detector section 102, vehicle velocity signal V output from the vehicle velocity detector section 105, etc., on the basis of which it generates a driving control signal SG1 for controlling the rotation of the steering assisting motor 103. The above-mentioned rack and pinion mechanism 108 etc. are accommodated in a gearbox 113 not shown in FIG. 38.
In short, the electric power steering apparatus 100 of FIG. 38 may be constructed by adding, to the construction of the traditional steering apparatus, the torque detector section 102, vehicle velocity detector section 105, control device 106, steering assisting motor 103 and power transmission device 104.
In the electric power steering apparatus 100, the steering assist torque, generated by the steering assisting motor 103 on the basis of the steering torque signal T, vehicle velocity signal V, etc., is boosted via the power transmission device 104 and delivered to a pinion shaft of the rack and pinion mechanism 108 so as to reduce steering torque to be manually applied by the driver. As the driver operates the steering wheel 101 to vary the traveling direction of the motor vehicle, a rotational force based on steering torque applied to the steering shafts 109a and 109b is converted into axial linear movement of the rack shaft 111, via the rack and pinion mechanism 108, to thereby vary the direction of the front road wheels 107 via the tie rods 112. During that time, the torque detector section 102, provided on the lower steering shaft 109b, detects the steering torque applied to the steering shaft 109b to generate an electric steering torque signal T, representative of the detected steering torque, that is supplied to the control device 106, and the vehicle velocity detector section 105 detects a traveling velocity of the vehicle steering to generate an electric vehicle velocity signal V that is supplied to the control device 106. Thus, on the basis of the steering torque signal T and vehicle velocity signal V, the control device 106 generates a motor current for driving the steering assisting motor 103, which in turn provides a steering assist force to the steering shafts 109b and 109a via the transmission mechanism 104. The motor 103 thus driven can reduce the steering force to be manually applied to the steering wheel 101 by the driver. If the steering torque is represented by “TH” and a coefficient of a steering assist amount AH is assumed to be a constant value “kA”, then AH=kA×TH.
Thus, if a load or pinion torque is represented by “TP”,
                                                        TP              =                            ⁢                              TH                +                AH                                                                                        =                            ⁢                              TH                +                                  kA                  ×                  TH                                                                                        (        1        )            Therefore, the steering torque TH can be expressed asTH=TP/(1+kA)  (2)Namely, the steering torque TH can be reduced to “pinion torque TP/(1+kA)”, where kA is equal to or greater than zero.
FIG. 39 shows detailed organization of mechanical and electric components in the electric power steering apparatus 100, where part of left and right end portions of the rack shaft 111 are shown in section. The rack shaft 111 is accommodated in a cylindrical housing 131, disposed in a widthwise direction (left-and-right direction of FIG. 39) of the vehicle, for axial sliding movement therein. Ball joints 132 are secured via screws to opposite ends of the rack shaft 111 that project beyond the housing 131, and left and right tie rods 112 are connected to the ball joints 132. The housing 131 has brackets 133 via which the housing 131 is secured to the body of the vehicle, and stoppers 134.at its opposite ends. In FIG. 39, reference numeral 135 represents an ignition switch, 136 an on-vehicle battery, and 137 an A.C. generator annexed to an engine of the vehicle. The A.C. generator 137 is activated to generate power in response to operation of the vehicle engine. Necessary electric power is supplied to the control device 106 from the battery 136 or A.C. generator 137. Further, reference numeral 138 represents a rack end that abuts against one of the stoppers 134 during axial movement of the rack shaft 138, and 139 a dust-sealing boot for protecting the interior of the gearbox from water, mud, dust, etc.
FIG. 40 is a sectional view taken along the A-A lines of FIG. 39, which clearly shows a structure for supporting the lower steering shaft 109b and detailed organization of the steering torque detector section 102, trans-mission mechanism 104 and rack and pinion mechanism 108. The lower steering shaft 109b is rotatably supported, via four bearings 141a, 141b, 141c and 141d, within a housing 113a forming the gearbox 113. The transmission mechanism 104 and rack and pinion mechanism 108 are also accommodated within the housing 113a, and the torque detector section 102 is secured to an upper portion of the housing 113a. The steering torque detector section 102 includes magnetostrictive films or coats 102b and 102c that are provided on the outer circumferential surface of the lower steering shaft 109b and surrounded by coils 102d, 102f, 102e and 102f and yoke section 102g; that is, the lower steering shaft 109b are surrounded by the coils 102d, 102f, 102e and 102f and yoke section 102g. The housing 113a has an upper opening closed with a lid 143 bolted thereto. The pinion 110 provided on a lower end portion of the lower steering shaft 109b is positioned between the bearings 141a and 141b. The rack shaft 111 is guided along a rack guide 145 and pressed against the pinion 110 via a compression spring 146. The power transmission mechanism 104 includes the worm gear 104a connected via a transmission shaft 148 to an output shaft of the steering assisting motor 103, and the worm wheel 104b secured to the lower steering shaft 109b. Specifically, the torque detector section 102, which is secured to the lid 143 in the steering gearbox 113, detects steering torque acting on the lower steering shaft 109b and outputs a value of the detected steering torque (steering torque signal) to the control device 106, which in turn supplies a motor signal to cause the motor 103 to generate appropriate steering assist torque.
The steering torque detector section 102 of the electric power steering apparatus 100 comprises a magnetostrictive torque sensor designed to directly detect steering torque applied to the steering shaft 109b, as compared to the traditional torque sensor that detects an twist or torsional angle of a torsion bar, converts the detected torsional angle into axial displacement and detects the converted axial displacement to thereby indirectly detect steering torque.
As illustrated in FIG. 40, the lower steering shaft 109b connected to the steering wheel 101 is rotatably supported, via the bearings 141c and 141d, within the gearbox 113, and two magnetostrictive coats, each in the form of a nickel-iron plating or the like, are provided on two, upper and lower, portions 102b and 102c of the outer surface between the bearings 141c and 141d. The magnetostrictive coats, each having a predetermined thickness, are imparted with opposite magnetic anisotropies and reverse magnetostrictive characteristics, as will be later described in relation to FIG. 40.
FIG. 41 is a diagram showing positional relationship among an exciting coil, detecting coils and magnetostrictive coats in the magnetostrictive torque sensor 102. The magnetostrictive coats 102b and 102c are formed, with a predetermined axial interval therebetween, on the surface of the lower steering shaft 109b, and the exciting coil 102f is disposed near the magnetostrictive coats 102b and 102c with a slight air gap left between the coil 102f and coats 102b and 102c. The exciting coil 102f is connected to an exciting voltage supply source 102h. Further, the detecting coil 102d is disposed near the magnetostrictive coat 102b with a slight air gap therebetween, while the detecting coil 102e is disposed near the magnetostrictive coat 102c with a slight air gap therebetween. When torque acts on the lower steering shaft 109b in the magnetostrictive torque sensor 102, the torque also acts on the magnetostrictive coats 102b and 102c, and reverse magnetostrictive effects are produced in the coats 102b and 102c in accordance with the applied torque. Thus, as a high-frequency A.C. voltage (exciting voltage) is supplied from the exciting voltage supply source 102h to the exciting coil 102f, magnetic field variation due to the reverse magnetostrictive effects of the coats 102b and 102c, based on the input torque, can be detected as variation in impedance or induced voltage. Then, the torque applied to the steering shaft 109b can be detected on the basis of the detected impedance or induced voltage variation.
Example of such reverse magnetostrictive characteristics is shown in FIG. 42, where the horizontal axis represents the steering torque while the vertical axis represents the impedance or induced voltage detected via the detecting coils when an A.C. voltage is applied to the exciting coil. Curve C100 represents variation in the impedance or induced voltage detected via the detecting coil 102d, and a curve C101 represents variation in the impedance or induced voltage detected via the detecting coil 102e. As indicated by the curve C100 corresponding to the detection via the detecting coil 102d, the impedance or induced voltage increases as the steering torque changes from a negative value to a positive value, takes a peak value P1 when the steering torque reaches a positive value T1, and decreases after the steering torque gets greater than the value T1. As indicated by the curve C101 corresponding to the detection via the detecting coil 102e, the impedance or induced voltage increases as the steering torque changes from a positive value to a negative value, takes a peak value P1 when the steering torque reaches a negative value −T1, and decreases after the steering torque gets smaller than the Value −T1. As shown, a steering-torque-vs.-impedance (induced voltage) characteristic obtained via the detecting coil 102d and a steering-torque-vs.-impedance (induced voltage) characteristic obtained via the detecting coil 102e present substantial mountain (upwardly-convex) curve shapes that are generally symmetrical with respect to the vertical axis, reflecting the opposite magnetic anisotropies of the upper and lower magnetostrictive coats 102b and 102c. Further, a straight line L100 represents a difference calculated by subtracting the characteristic curve C101, obtained via the detecting coil 102e, from the characteristic curve C100 obtained via the detecting coil 102d. The straight line L100 indicates that, ideally, the difference is zero when the steering torque is zero but varies linearly relative to variation in the steering torque within a steering torque range R. The magnetostrictive torque sensor uses particular regions or ranges of the characteristic curves C100 and C101 which present substantially constant gradients of sensitivity around a neutral torque point, so as to output detection signals corresponding to the direction and intensity of the input torque. Furthermore, using the characteristics of the straight line L100, the magnetostrictive torque sensor can detect the steering torque on the basis of the values detected via the detecting coils 102d and 102e. 
From Japanese Patent Publication No. 3268089, there is known a magnetostrictive torque sensor for detecting steering torque input to the steering shaft (rotational shaft), where a magnetostrictive coat is formed by first masking the surface of the rotational shaft and then performing an electroless plating process on the masked surface. However, in the case where an annular magnetostrictive coat is formed on the rotational shaft by first wrapping a masking tape on part of the surface of the rotational shaft to thereby mask the surface and then plating the masked surface as taught in the No. 3268089 patent publication, the magnetostrictive coat would have a greater thickness at its opposite axial end portions than the remaining coat portion, which would unavoidably deteriorate the detection accuracy due to reasons to be described later.
Further, FIGS. 43A-43E show a manner in which the magnetostrictive coats 102b and 102c are imparted with magnetic anisotropies in the conventional magnetostrictive torque sensor, according to which the steering shaft 109b is subjected to a plating process to form the magnetostrictive coats 102b and 102c (see FIG. 43A; however, only the magnetostrictive coat 102b is shown with the other magnetostrictive coat 102c omitted for clarity). After completion of the plating, counterclockwise twisting torque Tq is applied to an upper portion of the steering shaft 109b while clockwise twisting torque Tq is applied to a lower portion of the steering shaft 109b, to thereby impart stress to the circumferential surface of the steering shaft 109b (FIG. 43B). Then, with the twisting torque Tq kept applied, the magnetostrictive coats 102b and 102c are heated in a thermostatic bath (FIG. 43C) and then cooled (FIG. 43D). After the cooling, the twisting torque Tq is removed from the surface of the steering shaft 109b (FIG. 43E), and necessary sensor output setting is performed to manufacture a substantially complete steering shaft 109b. For details of such a magnetostrictive torque sensor manufacturing method, see Japanese Patent Application Laid-Open Publication No. 2002-82000.
In FIGS. 43A-43E, each circle or oval D100 depicted alongside a drawing of the steering shaft 109b represents an enlarged drawing of a minute portion of the magnetostrictive coat 102b, and arrows F1 and F2 represent a tension load and compressing load, respectively. Note that the “minute portion” is a model portion of the magnetostrictive coat assumptively extracted for the purpose of showing typical physical changes occurring in the magnetostrictive coat. In the step of FIG. 43B, the minute portion D100 of the magnetostrictive coat 102b is simultaneously subjected to the tension load F1 and compressing load F2, so that it is deformed into an oval shape with its longitudinal axis extending upward and rightward (i.e., in a lower-left-to-upper-right direction of the figure). In the step of FIG. 43C, undesired creep is produced in the magnetostrictive coat 102b due to the heating, and the minute portion D100 assumes a near-circular oval shape. FIG. 43D shows the minute portion D100 having shrunk after the cooling but still generally keeping the near-circular oval shape of FIG. 43C. Further, FIG. 43E shows a state where torsional torque acting in an opposite direction to the twisting torque Tq has been applied to the magnetostrictive coat 102b due to removal of the twisting torque Tq, and where the minute portion D100 has assumed an oval shape with its longitudinal axis extending upward and leftward, i.e. in a lower-right-to-upper-left direction of the figure.
Specifically, in the conventional magnetostrictive torque sensor, the magnetostrictive coat 102b is formed into a thickness of 40 μm, and the steering shaft 109b is subjected to the twisting torque Tq of 70 Nm, and then heated at a temperature in a range of 150-550° C. for 10-20 minutes with the twisting torque Tq still kept applied thereto.
However, the above-discussed conventional magnetostrictive torque sensor 102 has several drawbacks, such as instable detected values of the steering torque, great hysteresis, a considerably long time for heating the steering shaft (magnetostrictive coats) and poor productivity. FIG. 44 is a graph showing an example of actual reverse magnetostrictive characteristics of the conventional magnetostrictive torque sensor 102, where the horizontal axis represents the input steering torque while the vertical axis represents the impedance or induced voltage detected by the detecting coils when an A.C. voltage has been supplied to the exciting coil. In FIG. 44, a curve C102 represents a difference, i.e. (A-B) value, calculated by subtracting an actual characteristic curve obtained via the detecting coil 102e (corresponding to the curve C101 of FIG. 42) from an actual characteristic curve obtained via the detecting coil 102d (corresponding to the curve C100 of FIG. 42), and the curve C102 corresponds to the straight line L100 of FIG. 42. From FIG. 44, it can be seen that the curve C102 presents occurrence of a hysteresis instead of actually presenting a zero “(A-B)” value when the steering torque is zero. Therefore, in the case where such a magnetostrictive torque sensor is employed in an electric power steering apparatus, the magnetostrictive torque sensor would deteriorate a steering feel and thus can not be suitably put to practical use.
Further, the conventional magnetostrictive torque sensor suffers from another problem that the reverse magnetostrictive characteristics are susceptible to influences of characteristics of the magnetostrictive coat formed by the plating performed prior to the heating process, as shown in FIGS. 45A-45D comparatively illustrating measurements of the reverse magnetostrictive characteristics before and after the heating process. Specifically, FIGS. 45A and 45B illustrate the reverse magnetostrictive characteristics of the magnetostrictive coats 102b and 102c before the heating, where curves C110 and C111 represent variation in the impedance detected when clockwise torque was applied while curves C112 and C113 represent variation in the impedance detected when counterclockwise torque was applied. It can be seen that the magnetostrictive coat 102b presents a greater hysteresis than the other magnetostrictive coat 102c. Further, FIGS. 45C and 45D illustrate the reverse magnetostrictive characteristics of the magnetostrictive coats 102b and 102c detected when the coats 102b and 102c were heated at 300° C. for one hour. In the figures, curves C114 and C115 represent variation in the impedance detected when clockwise torque was applied while curves C116 and C117 represent variation in the impedance detected when counterclockwise torque was applied. It can be seen that the magnetostrictive coat 102b presents a greater hysteresis than the other magnetostrictive coat 102c. Namely, it was found that the magnetostrictive coat (e.g., 102b) having a relatively great hysteresis before the heating would present a great hysteresis even after the heating while the magnetostrictive coat (e.g., 102c) having a relatively small hysteresis before the heating would present a small hysteresis even after the heating; this means that the reverse magnetostrictive characteristics after the heating would be significantly influenced by the reverse magnetostrictive characteristics before the heating. Therefore, there has been a demand for a more sophisticated manufacturing method which can provide a magnetostrictive torque sensor capable of constantly achieving satisfactory torque detection with a small hysteresis without being influenced by characteristics of the magnetostrictive coats present before the heating.
Further, the magnetostrictive torque sensor, made by the above-discussed conventional manufacturing method, has the problem that its zero torque point and sensitivity would vary if it has been exposed for a long time in an engine room heated to a high temperature in the order of 80-100° C. FIG. 46 is a diagram showing high-temperature durability characteristics of the magnetostrictive torque sensor made by the above-discussed conventional manufacturing method. In FIG. 46, a characteristic curve C120 represents variation in the values detected by the detecting coil 102d at the beginning of actual use or operation of the magnetostrictive torque sensor, while a characteristic curve C121 represents variation in the values detected by the detecting coil 102e at the beginning of use of the magnetostrictive torque sensor. Characteristic curve C130 represents variation in the values detected by the detecting coil 102d after exposure, to the high temperature, of the sensor, while a characteristic curve C131 represents variation in the values detected by the detecting coil 102e after exposure, to the high temperature, of the sensor.
The characteristic curves C130 and C131, obtained through detection after the exposure to the high temperature (e.g., after the sensor has been used 1,000 times), each present a greater peak value of the impedance and a peak value of the input torque shifted toward the neutral torque point. By comparison between the characteristic curve C120 at the beginning of the use and the characteristic curve C130 after the exposure to the high temperature, it can be seen that there are a change in the peak impedance value from 26.6Ω to 26.9Ω, and a change in the peak impedance value from 45.1 Nm to 42.8 Nm. Such changes are due to a creep of the plating (which would remove distortion from the plating), and the characteristics at the beginning of the use can not be restored because the characteristics after occurrence of the creep are retained even after the cooling.
The above-mentioned changes result in a change or shift in the zero point Z200 to a zero point Z210. If such a zero point change occurs, detection values of the detecting coils 102d and 102e would exceed a predetermined range when a failure check is performed to determine presence of any failure in the sensor by ascertaining whether a sum of the detection values of the detecting coils 102d and 102e falls within the predetermined range; as a result, the failure check can not be performed appropriately.
Furthermore, with the conventional technique, where the detected torque value is determined on the basis of the difference between the characteristic curves obtained via the detecting coils 102d and 102e, the sensitivity of the torque sensor would undesirably vary as a gradient of the difference varies in accordance with the above-mentioned changes. Because the sensitivity of the torque sensor is generally set, during manufacture of the torque sensor, so as to achieve an optimal control amount of the torque, variation from the thus-set sensitivity may often lead to an uncomfortable steering feel. Further, if the neutral torque point is erroneously set with a deviation from the optimal point during manufacture of the torque sensor, then the torque sensor would present greater variation in the zero torque point and sensitivity.